Most well studied heme proteins (globins, c-and b-type cytochromes, peroxidases, P450) all have exactly the same heme prosthetic group yet each exhibits very different and very clearly defined functions. These differences are dictated by interactions between the protein and heme much of which has been deciphered through a variety of spectral probes and x-ray crystallography. The research in this proposal will center on testing hypotheses derived primarily from heme enzyme crystal structures. One question centers on the role that amino acid radicals play in peroxidase catalysis and how the protein environment helps to stabilize a particular Trp radical in cyctohrome c peroxidase. Other questions center on alternate binding sites for peroxidase substrates and mechanisms of inter-protein electron transfer. The primary methods to be employed are protein engineering coupled with crystallography and other biophysical approaches such as EPR spectroscopy and flash photolysis methods for following rapid reactions.